In recent years, a development in an artificial photosynthesis technology of electrochemically converting sunlight into a chemical substance in imitation of photosynthesis of plants is in progress from viewpoints of an energy problem and an environmental problem. That is because, for example, energy can be obtained sufficiently even if the sunlight is converted into the chemical substance in land such as desert whose utilization value is low and which is not utilized for producing plants and the chemical substance is transported to a distant place. The case of converting the sunlight into the chemical substance and storing it in a cylinder or tank has advantages that the storage cost of the energy can be reduced and the storage loss is small as compared with the case of converting the sunlight into electricity and storing it in storage batteries.
As a photoelectrochemical reaction device which electrochemically converts sunlight into a chemical substance, there has been known, for example, a two-electrode type device which includes an electrode having a reduction catalyst for reducing carbon dioxide and an electrode having an oxidation catalyst for oxidizing water (H2O), and in which these electrodes are immersed in water in which carbon dioxide is dissolved. At this time, the electrodes are electrically connected to each other via an electric wire or the like. In the electrode having the oxidation catalyst, H2O is oxidized by light energy, and thereby oxygen (1/2O2) is obtained and a potential is obtained. In the electrode having the reduction catalyst, by obtaining the potential from the electrode in which the oxidation reaction is caused, carbon dioxide is reduced and formic acid (HCOOH) or the like is produced. Thus, in the two-electrode type device, since a reduction potential of carbon dioxide is obtained by two-stage excitation, conversion efficiency from the sunlight to chemical energy is low.